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Photothermal Control of the Gelation Properties of Nickel Bis(dithiolene) Metallogelators under Near-Infrared Irradiation Kenny Mebrouk,† Sisir Debnath,† Marc Fourmigué, and Franck Camerel* Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes, France S Supporting Information *

ABSTRACT: The proper functionalization of nickel bis(dithiolene) complexes by pendant cholesteryl fragments allows for the formation of extended networks of intertwined fibers providing robust gels. Furthermore, such nickel bis(dithiolene) complexes are also efficient photothermal centers in solution in the near infrared (NIR), with a photothermal conversion efficiency comparable to that of gold nanoparticles. This unprecedented association in one single molecule, of the two properties, i.e., gelation ability and photothermal effect, gives a highly efficient handle to modulate the gel stability through light irradiation in the NIR region, providing a novel approach to photoresponsive gels.

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INTRODUCTION Molecular gels1 have emerged, during the past decade, as a fascinating class of soft and functional materials, which have found interest in a wide range of applications, such as confined reaction media,2−4 templates for well-defined inorganic materials,5−8 light-harvesting systems,9−12 sensors,13−15 biomaterials,16−19 and organic electronics.20,21 Their formation is generally described as a series of successive supramolecular selfassembly events giving rise to three-dimensional architectures more often consisting of three-dimensional networks of entangled fibers, which entrap the solvent molecules in large pockets. These supramolecular architectures constructed through noncovalent intermolecular interactions, including van der Waals, hydrogen-bonding, hydrophilic/hydrophobic segregation, or metal−metal interactions, are highly responsive to physical or chemical stimuli.22 Several types of environmental stimuli have been used to tune the gel properties, such as temperature, ionic strength, sonication, electric or magnetic fields, biological or chemical additives, etc. Especially, lightresponsive organogels emerge as new soft functional materials of great interest for potential applications in sensors and switches.23−26 Despite several progresses made in this field, photoresponsive organogels remain rare, and the development of light-responsive gelators is highly desirable for both fundamental and practical standpoints to develop new applications. Recently, a photocontrol of degelation processes was reported using gold nanoparticules embedded in peptide organogels.27 The well-known photothermal properties of the gold nanoparticules in the near infrared (NIR) were exploited there to disintegrate, upon laser illumination, the thermoresponsive peptide nanostructure responsible for the gel formation. However, an even more attractive route toward © XXXX American Chemical Society

such NIR photoresponsive gels would be the use of an inherently photoactive gelator, which would endorse simultaneously gelation properties and photothermal properties, avoiding aggregation/separation/durability problems encountered with nanoparticle dispersions in gels. In that respect, metal bis(dithiolene) complexes are known as strong NIR absorbers. They display, in their neutral state, high absorption coefficients (≈30 000 M−1 cm−1) in a wide range of NIR wavelengths, from 900 to 1600 nm, depending upon the choice of metal center and dithiolene substituents.28 Furthermore, these complexes are non-luminescent, implying that all of the absorbed energy is released in the environment as heat. This peculiar feature has found applications for laser printing on thin films.29,30 In addition, we have also recently demonstrated that the proper functionalization of nickel bis(dithiolene) cores with only four pendant cholesteryl fragments allows for the formation of metallogelators31,32 able to form robust chiral gels.33 Thus, supramolecular architectures, such as gels containing metal bis(dithiolene) complexes, should be highly responsive to laser irradiation in the NIR region, as demonstrated below. We report here a new series of organogelator (metallogelator) built around a nickel bis(dithiolene) and demonstrate that the photothermal properties of these complexes in the NIR region can be indeed efficiently used to control the degelation of the robust gels containing a thermoresponsive network of entangled fibers. Received: May 13, 2014 Revised: June 18, 2014

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dx.doi.org/10.1021/la501826j | Langmuir XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION

Materials and Reagents. Four nickel bis(dithiolene) complexes Ni8Cn (n = 3, 4, 6, and 11) appended with eight cholesteryl substituents were synthesized (see the Supporting Information for full details). 3,4-Dimethoxybenzaldehyde (99%), 3-bromopropanol (95%), 4-bromobutanol (95%), and cholesteryl chlorofomate (97%) were purchased from Sigma-Aldrich and used as received. 6-Bromo-1hexanol (97%), 11-bromoundecanol (97%), and NiCl2 (99.9%) were purchased from Alfa Aesar and used without further purification. DMI (99%), P4S10 (98%), and hydrobromic acid (48%) were purchased from Acros Organics. Anhydrous CH2Cl2 (Sigma-Aldrich), pyridine (Acros Organics), and triethylamine (Alfa Aesar) were obtained by distillation over P2O5 and KOH. The reactions were followed by thinlayer chromatography (TLC) plates, revealed by an ultraviolet (UV) lamp at 254 nm or phosphomolybdic acid. All other reagents and materials from commercial sources were used without further purification. Silica gel used in chromatographic separations was obtained from Acros Organics (silica gel, ultra pure, 40−60 μm). Apparatus and Methods. Nuclear magnetic resonance (NMR) spectra [300.1 (1H) and 75.5 MHz (13C)] were recorded on a Bruker Avance 300 spectrometer at room temperature using deuterated solvents as internal standards. Fourier transform infrared (FTIR) spectra were recorded using a Varian-640 FTIR spectrometer. UV− vis−NIR spectra were recorded using a Cary 5000 UV−vis−NIR spectrophotometer (Varian, Australia). Mass spectra were recorded with a Varian MAT 311 instrument by the Centre Régional de Mesures Physiques de l’Ouest, Rennes, France. Elemental analysis was performed at the Centre Régional de Mesures Physiques de l’Ouest, Rennes, France. Cyclic voltammetry were carried out on a 10−3 M solution of the complex in CH2Cl2, containing 0.2 M nBu4NPF6 as the supporting electrolyte. Voltammograms were recorded using an Autolab electrochemical analyzer (PGSTAT 30, Ecochemie BV). The reference electrode was saturated calomel electrode (SCE), and the counter electrode was graphite. Circular dichroism (CD) spectra were recorded on a Jasco-815 CD spectrometer. Differential scanning calorimetry (DSC) was carried out using a NETZSCH DSC 200 F3 instrument. Atomic force microscopy (AFM) measurements were performed using a Veeco MultiMode with NanoScope IIID controller scanning probe microscope. First, Ni8C4 and Ni8C6 gels were prepared in hexane at 20 mg mL−1. Then, the gel was diluted 10 times before preparing the samples on a mica surface. The surfaces were dried for 20 h under vacuum before imaging. X-ray scattering experiments were performed using a FR591 Bruker AXS rotating anode X-ray generator operated at 50 kV and 50 mA with monochromatic Cu Kα radiation (λ = 1.541 Å) and point collimation. The samples were held in Lindemann glass capillaries (1 mm diameter). The patterns were collected with a Mar345 image plate detector (Marresearch, Norderstedt, Germany). For photothermal studies, 1 mL solutions containing different amounts of complexes were irradiated through a quartz cuvette with a 980 nm wavelength semiconductor laser for 10 min. The power intensity of the laser could be adjusted externally (0−1.03 W). The output power was independently calibrated using a optical power meter. A thermocouple with an accuracy of ±0.1 °C was inserted into the solution. The thermocouple was inserted at such a position that the direct irradiation of the laser was avoided. The temperature was measured every 30 s. Laser-triggered degelation of Ni8Cn organogels was investigated in glass vials using the same NIR laser.

Figure 1. Structure of the nickel bis(dithiolene) complexes Ni8Cn (n = 3, 4, 6, and 11) and their UV−vis−NIR spectra in dichloromethane (C = 10−5 mol L−1).

region, centered at 950−960 nm, characteristic of metal bis(dithiolene) cores (Figure 1 and see Table S1 of the Supporting Information). Note that the shape and maximum of this strong NIR absorption band are weakly affected in the solid state or gelled state.13 Only a slight broadening and a small hypsochromic shift of the absorption were observed between the solution of Ni8C6 in dichloromethane (959 nm) and the gel of Ni8C6 in dodecane (945 nm) (Figure 2).

Figure 2. Comparison of the UV−vis−NIR spectra of a solution of Ni8C6 in CH2Cl2 solution and a gel of Ni8C6 in dodecane.

Cyclic voltammetry of the neutral complexes in CH2Cl2 shows two reversible reduction processes centered at −0.07 and −0.85 V, corresponding to the formation of the monoanionic and dianionic species, respectively. The redox potentials are weakly affected by the variation of the chain length of the spacer (see Table S1 of the Supporting Information). The strong NIR absorption of the Ni8Cn complexes at 950− 960 nm motivated us to explore their photothermal activities, first in solution, with a 980 nm wavelength laser. The temperature elevation of decane suspensions at different concentrations (0−40 mg L−1) was measured under laser irradiation with a power of 1.03 W for 10 min (power surface density, 1.62 W cm−2; spot size, 9 mm) (Figure 3a). In pure decane, a slight increase of 4 °C was observed after 10 min of irradiation, whereas an obvious concentration-dependent

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RESULTS AND DISCUSSION Four different complexes with eight cholesteryl substituents, Ni8Cn (n = 3, 4, 6, and 11), were considered, depending upon the length of the spacer between the dithiolene core and cholesteryl fragments (Figure 1). Full experimental details of the synthetic process are presented in the Supporting Information. The UV−vis−NIR absorption spectra of the Ni8Cn complexes measured in chloroform solution at room temperature display the strong absorption band in the NIR B


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increase was observed after addition of Ni8C4 under laser irradiation. Below 0.05 mg mL−1, no noticeable photothermal effects have been detected. The temperature change (ΔT) drastically goes up with an increase of the complex concentration from 0.05 to 10 mg L−1 and then reaches a maximum (+48.1 °C). At higher concentrations, however, the temperature elevation slightly decreases (panels a and b of Figure 3). This phenomenon is likely attributed to a shallow penetration of the laser inside the solution (saturated absorption) inducing a longer heat diffusion time toward the thermocouple coupled to a fast heat lost. As expected, the temperature variations increase when the irradiation power density increases. Measurements performed on equimolar solutions of Ni8C4 and Ni8C6 in decane for 10 min at different NIR power densities show that the temperature elevation is higher with Ni8C4 (Figure 3c) and that the temperature difference between the two solutions also increases for higher laser power densities. This dissimilarity between Ni8C4 and Ni8C6 is attributed to the slight absorption difference observed on the UV curves (Figure 1). The photothermal conversion efficiency (η) was evaluated on the solution of Ni8C4 in decane at 0.1 mg mL−1 [absorbance at 980 nm (A980) = 0.747 23] by monitoring the complete temperature profile under continuous irradiation at 980 nm laser with a power of 1.03 W until 10 min and after turning off the laser for 10 min (Figure 4). The η value was calculated

Figure 4. Temperature profile of a solution of Ni8C4 in decane at 0.1 mg mL−1 when illuminated with a 980 nm laser (power, 1.03 W; spot size, 9 mm) for 10 min and after turning off of the laser for 10 min. (Inset) Time constant for heat transfer is determined by applying the linear time from the cooling period (from 600 to 1200 s) versus the negative natural logarithm of the driving force temperature.

according to the following equation described by Roper et al. based on the energy balance of the system:34 Figure 3. (a) Temperature elevation of solutions of Ni8C4 in decane at various concentrations (0−40 mg mL−1) as a function of time (0− 600 s) under irradiation with a 980 nm wavelength laser at a power of 1.03 W (spot size, 9 mm; power surface density, 1.62 W cm−2). (b) Presentation of ΔTmax as a function of the concentration of Ni8C4 in decane in mg mL−1 and the power of the 980 nm wavelength laser after 10 min of irradiation (spot size, 9 mm). (c) Comparison of the temperature elevation of two solutions of Ni8C4 (4.0 mg mL−1) and Ni8C6 (4.2 mg mL−1) in decane at 0.9 mmol L−1 under 980 nm laser irradiation at 0.30, 0.67, and 1.03 W for 10 min (spot size, 9 mm).

η = (hSΔTmax − Q decane)/I(1 − 10−A 980)

(1)

where h is the heat-transfer coefficient, S is the surface area of the container, ΔTmax is the maximum steady-state temperature change of the Ni8C4 solution, I is the power of the laser, and A980 the absorbance at 980 nm (A980 = 0.747 23 for Ni8C4 in decane at 0.1 mg mL−1). Qdecane was measured independently and represents heat dissipated from light absorbed with a pure decane solution. The hS value is derived according to eq 2 C


Langmuir τs = mdecaneCdecane/hS

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(2)

where τs is the sample system time constant and mdecane (0.73 g) and Cdecane (2.21 J g−1) are the mass and heat capacity of decane, respectively. τs is given be the slope of the linear fitting from the time of the laser off state versus −ln(ΔT/ΔTmax) (inset of Figure 4). According to this equation, the η value determined for Ni8C4 is ∼19% and compares well to the η values obtained with gold nanostructures (13−22%), which are currently the object of great interest for photothermal therapy.35 NMR experiments and TLC performed after NIR irradiation did not evidence any degradation of the complexes, confirming their remarkable photothermal stability,29,30 even after exposure to laser irradiation for a long time, in contrast to gold nanostructures. Thus, one can already conclude that metal bis(dithiolene) complexes can efficiently convert the 980 nm wavelength laser energy into heat energy. Gelation ability of Ni8Cn (n = 3, 4, 6, and 11) has been evaluated in various polar, nonpolar, protic, and nonprotic solvents by the “stable to inversion of a test tube” method (see Table S2 of the Supporting Information). While Ni8C3 and Ni8C11 were found to be inefficient toward gel formation, Ni8C4 and Ni8C6 were found to be good gelators of linear alkanes from C6 to C12. Heating Ni8C4 and Ni8C6 in linear alkane chain solvents gives homogeneous, fluid solutions, which provide robust gels upon cooling, displaying a thermally reversible sol−gel phase transition (Figure 6a). The temperatures of the sol−gel transition were found to depend upon mainly the nature of the complex (∼35 °C with Ni8C4 and ∼50 °C with Ni8C6) but not the nature of the gelled solvent (determined by DSC; see Figures S2 and S3 of the Supporting Information). It should be noticed that these octa-substituted compounds are slightly less efficient toward gelation than their tetra-substituted counterparts, likely attributed to a higher number of packing constraints.33 The complete volume of solvent is immobilized and can support its own weight without collapsing. Surprisingly Ni8C3 and Ni8C11 were found to be soluble in linear alkane chain solvents and fail to form gel. This can be due to an imbalance between hydrophobic−hydrophilic and π−π stacking interactions or odd−even effects introduced by the linear spacer. AFM examination of diluted gels of Ni8C4 and Ni8C6 complexes in hexane, deposited by spin coating on mica, confirms the presence of fibrillar assemblies (Figure 5 and see Figure S4 of the Supporting Information). The formation of these dense 3D networks of interlocked fibers is at the origin of the gelation of solvents. The average diameter of the fibers is relatively homogeneous and slightly increases from C4 to C6 (∼50 nm for Ni8C4 and ∼60 nm for Ni8C6). The mean diameter observed for the fibers is large compared to the molecular dimension, thereby suggesting that the xerogels are constituted of bundles of thinner intertwined molecular chains. The X-ray diffraction (XRD) pattern obtained on a xerogel of Ni8C6 in dodecane displays three broad diffraction peaks in the small angle region in the ratio 1:∼√3:∼√7, attributed to some short-range ordering of extended molecular aggregates in a twodimensional (2D) hexagonal lattice (see Figure S5 of the Supporting Information). Two broad diffraction peaks are also visible in the wide angle region at 5.6 and 4.7 Å and are associated with some local molecular interactions between the cholesteryl fragments and the carbon chains, respectively. The formation of the fibrillar aggregates is likely directed by stacking

Figure 5. AFM tapping mode image (5 × 5 μm) of a dried diluted gel of Ni8C4 in hexane (C = 2 mg mL−1) deposited on mica. The gels have been diluted 5 times (diluted gel) with hexane, to disperse the aggregates and, hence, to facilitate the observation of the individual fibers.

between the metal bis(dithiolene) cores and van der Waals interactions between the cholesteryl groups at the periphery, as observed in the formation of columnar mesophases.36 CD measurements have also revealed that, unlike their tetrasubstituted counterparts, these fibers are not chiral.33 To investigate how the photothermal properties of the metal bis(dithiolene) cores can be indeed used to destructure the thermosensitive nanoarchitectures at the origin of the gel formation, gels were irradiated with a NIR laser. Figure 6b presents the photographs of the organogels before and after laser exposure. Before laser exposure, as already mentioned, the gel can support its own weight without collapsing. However, after 1−2 min of irradiation, the gels start to melt at the impact of the laser on the front of the vial. Upon longer exposure times, the laser gradually passes through the gel and more and more solution flows down the vial. The gel is almost completely decomposed after 5 min of irradiation, because of the aforementioned photothermal properties of Ni8C6 gelators. The kinetics of this degelation process depends upon the concentration of the gelator inside the gels and the power of the laser used. The resulting liquid mixture slowly reformed the organogel, when the laser exposure was stopped and the solution was allowed to cool. The same behavior was observed in decane and dodecane with Ni8C4 and Ni8C6 (see Figure S6 of the Supporting Information). These experiments demonstrate that the photothermal properties of the nickel bis(dithiolene) complexes can be used to tune their gelation properties. From the measurements performed on solutions, it can be observed that, at 0.370 W, the temperature increase is not sufficient to reach the sol−gel transition (ΔT ∼ 14 °C), but this indicates that the temperature increase generated under irradiation in the vicinity of the complex fibers, before diffusion toward the solvent, is sufficient to disintegrate the supramolecular organization.

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CONCLUSION In conclusion, this work demonstrates quantitatively that nickel bis(dithiolene) complexes are efficient photothermal centers in solution. They can be very good alternatives to know nanoparticulate systems for photothermal therapies (metal or carbon nanostructures) because these molecular systems are D


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Figure 6. Heat- and laser-controlled reversible sol−gel transition with Ni8C6 in heptane (48 mg L−1), with a laser power of 0.37 W (absorbance, 980 nm; spot size, 7 mm; power surface density, 0.96 W cm−2). (4) Guler, M. O.; Stupp, I. A self-assembled nanofiber catalyst for ester hydrolysis. J. Am. Chem. Soc. 2007, 129, 12082−12083. (5) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Organic gels are useful as a template for the preparation of hollow fiber silica. Chem. Commun. 1998, 1477−1478. (6) Sone, E.D.; Zubrarev, E. R.; Stupp, S. I. Semiconductor nanohelices templated by supramolecular ribbons. Angew. Chem., Int. Ed. 2002, 41, 1705−1709. (7) Jung, J. H.; Lee, S.-H.; Yoo, J. S.; Yoshida, K.; Shimizu, T.; Shinkai, S. Creation of double silica nanotubes by using crownappended cholesterol nanotubes. Chem.Eur. J. 2003, 9, 5307−5313. (8) Kobayashi, S.; Hanabusa, K.; Hamasaki, N.; Kimura, M.; Shirai, H.; Shinkai, S. Preparation of TiO2 hollow-fibers using supramolecular assemblies. Chem. Mater. 2000, 12, 1523−1525. (9) Nakashima, T.; Kimizuka, N. Light-harvesting supramolecular hydrogels assembled from short-legged cationic L-glutamate derivatives and anionic fluorophores. Adv. Mater. 2002, 14, 1113−1116. (10) Sugiyasu, K.; Fujita, N.; Shinkai, S. Visible-light-harvesting organogel composed of cholesterol-based perylene derivatives. Angew. Chem., Int. Ed. 2004, 43, 1229−1233. (11) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Gelation-assisted light harvesting by selective energy transfer from an oligo(pphenylenevinylene)-based self-assembly to an organic dye. Angew. Chem., Int. Ed. 2003, 42, 332−335. (12) Ren, Y.; Kan, H.; Thangadurai, V.; Baumgartner, T. Bio-inspired phosphole-lipids: From highly fluorescent organogels to mechanically responsive FRET. Angew. Chem., Int. Ed. 2012, 51, 3964−3968. (13) Koshi, Y.; Nakata, E.; Yamane, H.; Hamachi, I. A fluorescent lectin array using supramolecular hydrogel for simple detection and pattern profiling for various glycoconjugates. J. Am. Chem. Soc. 2006, 128, 10413−10422. (14) Mukhopadhyay, P.; Iwashita, Y.; Shirakawa, M.; Kawano, S.-I.; Fujita, N.; Shinkai, S. Spontaneous colorimetric sensing of the positional isomers of dihydroxynaphthalene in a 1D organogel matrix. Angew. Chem., Int. Ed. 2006, 45, 1592−1595. (15) Yoshimura, I.; Miyahara, Y.; Kasagi, N.; Yamane, H.; Ojida, A.; Hamachi, I. Molecular recognition in a supramolecular hydrogel to afford a semi-wet sensor chip. J. Am. Chem. Soc. 2004, 126, 12204− 12205. (16) Silva, G. A.; Czeisler, C.; Niece, K. L.; Beniash, E.; Harrington, D. A.; Kessler, J. A.; Stupp, S. I. Selective differentiation of neural progenitor cells by high-epitope density nanofibers. Science 2004, 103, 1352−1355. (17) Van Bommel, K. J. C.; Stuart, M. C. A.; Feringa, B.L.; Van Esch, J. Two-stage enzyme mediated drug release from LMWG hydrogels. Org. Biomol. Chem. 2005, 3, 2917−2920. (18) Vemula, P. K.; Li, J.; John, G. Enzyme catalysis: Tool to make and break amygdalin hydrogelators from renewable resources: A delivery model for hydrophobic drugs. J. Am. Chem. Soc. 2006, 128, 8932−8938. (19) Jayawarma, V.; Ali, M.; Jowitt, T. A.; Miller, A. F.; Saiani, A.; Gough, J. E.; Ulijn, R. V. Nanostructured hydrogels for threedimensional cell culture through self-assembly of fluorenylmethoxycarbonyl−dipeptides. Adv. Mater. 2006, 18, 611−614.

easily functionalized, as shown here. The unprecedented association of gelation ability and photothermal effect in one single molecule provides a handle to modulate the gel stability through light irradiation in the NIR region. Such photoresponsive gels under laser illumination will offer new opportunities in advanced nanotechnological applications. Studies are currently under progress in our laboratory to design biocompatible hydrogelators based on metal bis(dithiolene) complexes for the control of drug deliveries under irradiation in the NIR region.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, synthesis, table containing the optical and electrochemical properties of the proligands and complexes, UV−vis−NIR spectra of the proligands, table containing the gelation test results, DSC curves, AFM images, and time evolution of a Ni8C6/dodecane gel under NIR irradiation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

Kenny Mebrouk and Sisir Debnath contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The French National Centre for Scientific Research (CNRS), the French “Ministère de l’Enseignement Supérieur et de la Recherche”, the University of Rennes 1, and the “Région Bretagne” are gratefully acknowledged for their financial support. The authors also thank the “Institut de Physique de Rennes” to give us access to the laser facilities. Cristelle Mériadec and Franck Artzner are also gratefully acknowledged for their help in XRD experiments.

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REFERENCES

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